The discovery of a second oestrogen receptor (ER), ERβ, has drastically changed our view of oestrogen action. Since the two ERs, ERα and ERβ, have somewhat different tissue and cellular distribution as well as ligand binding specificity the possibility exists that they have different biological roles. Indeed, several observations seem to indicate that they may even have opposite effects so that ERβ diminishes the activity of ERα. The CNS contains both ERα and ERβ and it is conceivable that they may have specific and individual roles in oestrogen signalling in the brain.